1. Field of the Invention
The present invention relates to a radiation-heated fluidized-bed reactor and to a process for producing high-purity polycrystalline silicon by means of this reactor.
2. The Prior Art
High-purity polycrystalline silicon is used, inter alia, as a starting material for the production of electronic components and solar cells. It is obtained by thermal decomposition of a silicon-containing gas or a silicon-containing gas mixture. This process is known as chemical vapor deposition (CVD). This process is carried out on a large scale in so-called Siemens reactors.
Recently, however, there have been wide-ranging efforts to utilize a fluidized-bed process as an alternative to the discontinuous Siemens process. In this case, a fluidized bed of silicon particles, for example approximately spherical particles with a diameter of 200 m–3000 μm is used. The particles are heated to the required deposition temperatures of preferably 600–1100° C., and a silicon-containing gas or gas mixture, for example trichlorosilane or a trichlorosilane/hydrogen mixture, is passed through the fluidized bed. In the process, elemental silicon is deposited on the silicon particles and the size of the individual particles grows. By regularly withdrawing grown particles and adding smaller silicon particles as seed particles, it is possible for the process to be operated continuously, with all the associated advantages.
A significant difficulty in the fluidized-bed process is the introduction of the energy in order for the fluidized bed to be operated at the high temperatures required, which are preferably between 600 and 1100° C. The deposition reaction is not selective with regard to the material of the solid surface, and the CVD reaction preferentially takes place on the hottest surface. If the energy is supplied to the fluidized bed by a wall heater system, the wall of the fluidized bed is the hottest surface in the reaction chamber and there is a correspondingly high deposition of silicon on this wall. As a result of silicon growing on continuously, this wall layer may considerably impair the heating, possibly even making it unable to function. Accordingly, various methods are known in the prior art aimed at avoiding this situation.
WO 96/41036 describes a process in which the energy is introduced through the gas supply by means of a highly bundled light beam. The light beam penetrates through the gas, is absorbed by the silicon particles and heats the latter. A drawback of this process is that only that area of the fluidized bed which is in the immediate vicinity of the entry of the silicon-containing gas is heated.
The heating of the fluidized bed by means of microwaves is known from DE 3638931 C2 (corresponds to U.S. Pat. No. 4,786,477). Microwaves are fed to the fluidized bed via a microwave-permeable fluidized-bed wall made from quartz. As a result, the particles are directly heated and are therefore hotter than the wall. However, since the wall/particles heat transfer ensures that the wall/particles temperature difference is only slight, in this case too there is undesirable deposition of silicon on the wall.
Therefore, in DE 4327308 C2 (corresponds to U.S. Pat. No. 5,382,412), microwave heating was developed further and the fluidized bed was vertically divided into a lower heating zone and a reaction zone situated above it. In the heating zone, the particles are fluidized by an inert gas, preferably hydrogen, and are heated by means of microwaves. As a result of particle and gas convection, the reaction zone above it is heated to the deposition temperature. The silicon-containing gas is initially added to the reaction zone via a nozzle. This is when the deposition reaction takes place. The heating zone itself remains free from wall deposition and the microwave heating is therefore not impaired even after prolonged operation.
However, the temperature-dependent way in which microwaves are introduced into silicon and the fact that the introduction of energy is dependent on the geometry of the reactor and the supply of microwaves, a reactor of this type leads to an energy supply which is uneven over a large area. In the specialist field, the resulting problem is known as hot spots/cold spots and is mentioned, for example, in U.S. Pat. No. 4,967,486 in connection with a microwave-heated fluidized bed. The result is that some silicon particles are excessively overheated and particles sinter together, and, in addition, relatively large particle agglomerates are formed in the fluidized bed. These silicon agglomerates are undesirable in the product and represent a considerable disruption to operation of the reactor, since they have relatively poor flow properties.
Also, particles were found to adhere to the fluidized-bed wall and were in some cases heated to such a point that they fused on (T>1400° C.). Moreover, the considerable overheating of particles in the immediate vicinity of the waveguide terminals leads to an excessive thermal load on the fluidized-bed walls. In combination, the drawbacks listed lead to unstable operation and an unsatisfactory product quality. Although the fluidizing of the fluidized bed and therefore the mixing behavior has a compensating effect in terms of the temperature distribution in the fluidized bed, this effect is highly dependent on the level of fluidization.
The higher the gas velocity, the stronger the vertical and horizontal mixing of particles. However, increasing the gas velocity to well beyond the fluidization velocity umf, characterized for example by equation (18) Chapter 3 in “Fluidization Engineering”; D. Kunii, O. Levenspiel; Butterworth-Heinemann; Second Edition 1991:
                    1.75                              ɛ            mf            3                    ⁢                                          ⁢                      ϕ            s                              ⁢                          ⁢                        (                                                    d                p                            ⁢                                                          ⁢                              u                mf                            ⁢                                                          ⁢                              ρ                g                                      μ                    )                2              +                            150          ⁢                                          ⁢                      (                          1              -                              ɛ                mf                                      )                                                ɛ            mf            3                    ⁢                                          ⁢                      ϕ            s            2                              ⁢                          ⁢              (                                            d              p                        ⁢                                                  ⁢                          u              mf                        ⁢                                                  ⁢                          ρ              g                                μ                )              =                    d        p        3            ⁢                          ⁢              ρ        g            ⁢                          ⁢              (                              ρ            s                    -                      ρ            g                          )            ⁢                          ⁢      g              μ      2      where                εmf proportion of voids at the fluidization point        øs sphericity of the particles        dp particle diameter        ρg gas density        ρs solids density        μ dynamic viscosity of the gas        g acceleration due to gravityalways leads to an increase in the energy supply required. This is because the fluidizing gas generally flows to the fluidized bed at a temperature which is significantly lower than that of the particles and is heated to approximately the temperature of the fluidized bed as it flows through the bed. Therefore, although an increase in the gas flow rate can counteract the formation of hot spots/cold spots, it always leads to an increased energy consumption by the process.        